1. Field of the Invention
The present invention relates to a ceramic package mounted with a crystal oscillator functioning as a frequency generator and a frequency modulator. In particular, the structure of the ceramic package is improved in order to reduce its size while minimizing thermal influence to a crystal wafer mounted on the ceramic package.
2. Description of the Related Art
A crystal oscillator is generally used in various forms such as a frequency generator, a frequency modulator and a frequency converter. The crystal oscillator utilizes crystal having excellent piezoelectric characteristics, in which crystal functions as a stable mechanical vibrator.
Crystal is artificially grown in a high pressure autoclave, and cut along a crystal axis into pieces of various sizes and shapes. A crystal piece is formed into the shape of a wafer. The crystal wafer is necessarily formed to have a low phase noise, a high Q value and a low change rate of frequency according to time and environmental change.
The crystal wafer is fixed in a package and electrodes are formed on the crystal water for electrical connection so as to be used as a crystal oscillator. The crystal wafer is electrically connected to external electric devices and bonded to the package via conductive adhesive to this end, in which sufficient bonding areas are needed to obtain excellent vibration efficiency of the crystal oscillator and its reliability against external impact.
Sealing is provided to protect the crystal oscillator fixed to the package from external circumstances and contaminating substances. Since the operating efficiency of the crystal oscillator and its quality are greatly influenced by external environmental change and contamination, the crystal package is preferably so sealed to have a very low leak rate. To this end, a metallic lid-supporting sheet is bonded to a top of the ceramic package, and a lid made of the same material as the lid-supporting sheet is placed on the lid-supporting sheet and sealed thereto via electric welding. It is important that ceramic-to-metal and metal-to-metal bonding areas are airtight since external contaminating substances penetrated into the package may deteriorate several characteristics of the crystal oscillator such as reliability.
Peripheral devices are getting rapidly reduced in their sizes as personal portable terminals and radio apparatuses are miniaturized owing to ongoing development of mobile and radio communication apparatuses. On the other hand, the capacity of the crystal oscillator is enlarged, compared to those of the peripheral devices. Spatial restraints against the crystal oscillator are increased since electrical and mechanical connection of the crystal oscillator to the outside is restricted and miniaturization of the crystal wafer is limited. In particular, a crystal oscillator mounted on a temperature compensated crystal oscillator or TCXO package increases overall volume, and thus miniaturization of this crystal oscillator is getting more necessary. Accordingly, the art growingly requires a technique for miniaturizing the crystal oscillator and reducing its size.
FIG. 1A and FIG. 1B are sectional and plan views of a conventional ceramic package for a crystal oscillator. In FIGS. 1A and 1B, the crystal oscillator ceramic package comprises a bottom sheet 11 constituting a bottom of the package and a buffer sheet 12 disposed on the bottom sheet 11 for supporting a crystal wafer 16. The package also comprises an insulating sheet 13 on the buffer sheet 12 for obtaining a vibration space of the crystal oscillator as well as insulating the buffer sheet 12. The bottom sheet 11, the buffer sheet 12 and the insulating sheet 13 are commonly made of ceramic. Electrodes 18 are applied to a top of the buffer sheet 12 for electric connection of the crystal oscillator. The buffer sheet 12 serves to ensure stable oscillation of the crystal and protect the crystal wafer from external impact, and the electrodes function to connect between the crystal wafer and external terminals. The crystal wafer 16 is attached to the electrodes 18 of the buffer sheet 12 via conductive adhesive 19 and electrically connected to the same. A lid-supporting sheet 14 is formed on a top of the insulating sheet 13 to function as a cover of the ceramic package, and a lid 15 is formed on the lid-supporting sheet 14 for insulation of the ceramic package.
Since the conventional crystal oscillator ceramic package shown in FIGS. 1A and 1B comprises total five sheets from the bottom sheet to the uppermost lid, it is difficult to reduce the size of the ceramic package. Accordingly, the art has studied another crystal oscillator ceramic package with a structure improved over that of the above-described ceramic package.
FIG. 2 is a sectional view of another conventional ceramic package for a crystal oscillator. Referring to FIG. 2, buffer sheets 22 and 22xe2x80x2 are formed on a bottom sheet 21 in the form of W or Mo deposited films at a thickness of about 10 xcexcm. A top of the buffer sheet 22 is plated with metal such as Au, and a crystal wafer 26 is attached on the buffer sheet 22 via conductive adhesive 29. An insulating sheet 23 is disposed around a top of the bottom sheet 21 to form a wall, and a supporting sheet 24 is formed on the insulating sheet 23 for supporting a lid 25. The insulating sheet 23 is made of insulating ceramic to function as an insulator between the buffer sheet 22 and the supporting sheet 24, and the supporting sheet 24 is made of a metallic material substantially identical with the lid 25.
The crystal oscillator ceramic package having the structure shown in FIG. 2 is reduced in the number of laminated sheets compared to the conventional ceramic package shown in FIGS. 1A and 1B. However, the bottom sheet is formed thicker than that of the ceramic package in FIGS. 1A and 1B. That is, since the thin buffer sheet is disposed on the bottom sheet and the crystal is attached on the buffer sheet, the bottom sheet is necessarily formed thick to protect the crystal from external impact or fracture, thereby causing difficulty in forming a ceramic package. Also, the thin buffer sheet affords poor stability to the crystal against external impact.
FIG. 3 is a sectional view of further another conventional ceramic package for a crystal oscillator disclosed in a Japanese Laid-Open Patent Application Serial No. 2000-124765, which pertains to a crystal oscillator and a fabricating process thereof. Referring to FIG. 3, the crystal oscillator comprises a crystal wafer 36 having electrodes, a supporting portion 22 for supporting the crystal wafer 36, a first substrate 31 having conductor means for electrically connecting between the electrodes of the crystal wafer 36 and an external circuit, a second substrate 35 functioning as a cover and an insulating sheet 23 forming a side wall to enclose the crystal wafer 36. The crystal oscillator also comprises glass balls 24 in the insulating sheet 23.
In the crystal oscillator ceramic package shown in FIG. 3, the insulating sheet 23 is made of resin and melted to seal the package. Because the insulating sheet 23 made of resin may not uniformly maintain its height when it is melted, the glass balls 24 are previously disposed around the first substrate 31 before the insulating sheet 23 is melted in order tQ maintain the height of the insulating sheet 23. The glass balls 24 are inevitably used in such a structure, thereby disadvantageously increasing the manufacturing cost of an article. Melting the insulating sheet 23 produces gas or fume to contaminate the crystal wafer or an inner region of the package thereby deteriorating the quality and reliability of the article.
The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to improve the structure of a ceramic package for a crystal oscillator in order to further miniaturize the ceramic package.
It is another object of the invention to reduce the area of an electrode contacting with a crystal in order to decrease the floating capacitance of a crystal container. Also the invention reduces the number of ceramic sheets of a package to reduce heat capacity thereby saving welding power for sealing a lid functioning as a cover while reducing the influence of heat from welding to the crystal.
It is yet another object of the invention to provide a miniaturized ceramic package which can reduce overall flection while obtaining sufficient strength.
According to an aspect of the invention to obtain the above objects, a ceramic package for a crystal oscillator comprises: a bottom sheet having an external terminal; a buffer sheet disposed on a periphery of the bottom sheet and having an internal terminal on a top thereof, the internal terminal being electrically connected to the external terminal in the bottom sheet; a crystal wafer vibratably mounted on the buffer sheet and having an electrode electrically connected to the internal terminal on the buffer sheet; a supporting sheet formed on a periphery of the buffer sheet and spaced from the internal terminal on the buffer sheet at a designated interval; and a lid covering the supporting sheet for sealing a portion of the ceramic package for receiving the crystal wafer.
Preferably, the supporting sheet and the lid are made of metal, and the ceramic package for a crystal oscillator of the invention may further comprise a W bonding layer between the bottom sheet and the buffer sheet for bonding the bottom and buffer sheets together.
Preferably, the crystal wafer is bonded to a top portion of the buffer sheet via conductive adhesive, wherein the internal terminal on the buffer sheet has a recess on a central portion thereof, whereby the conductive adhesive is applied about the recess. Also preferably, the ceramic package for a crystal oscillator of the invention may further comprise via holes in the bottom and buffer sheets for electrically connecting the external terminal in the bottom sheet to the internal terminal on the buffer sheet.
According to another aspect of the invention to obtain the above objects, a ceramic package for a crystal oscillator comprises: a bottom sheet having an external terminal in an underside thereof; a buffer sheet disposed on a periphery of the bottom sheet and having an internal terminal on a top thereof, the internal terminal being electrically connected to the external terminal in the bottom sheet; a crystal wafer vibratably mounted on the buffer sheet adjacent to the internal terminal via conductive adhesive and having an electrode electrically connected to the internal terminal on the buffer sheet; a supporting sheet formed on a periphery of the buffer sheet to expose a first lateral portion of the buffer sheet where the internal terminal is disposed and a second lateral portion of the buffer sheet opposite to the first lateral portion; and a lid covering the supporting sheet for sealing a portion of the ceramic package for receiving the crystal wafer, wherein the internal terminal on the buffer sheet is spaced from the supporting sheet at a designated interval.
Preferably, the ceramic package for a crystal oscillator of the invention further comprise a W bonding layer between the bottom sheet and the buffer sheet for bonding the bottom and buffer sheets together, wherein the internal terminal on the buffer sheet has a recess on a central portion thereof, whereby the conductive adhesive is applied about the recess. Preferably also, the ceramic package for a crystal oscillator of the invention further comprise via holes in the bottom and buffer sheets for electrically connecting the external terminal in the bottom sheet to the internal terminal on the buffer sheet.